1. Field of the Invention
This invention relates generally to centrifuges, and more particularly to decanting centrifuges with a rotating bowl and scroll.
2. Description of Related Art
The prior art discloses a variety of decanter centrifuges or xe2x80x9cdecantersxe2x80x9d which, in many embodiments, include a rotating centrifuge bowl rotating at one speed and in which a screw conveyor (xe2x80x9cscrollxe2x80x9d) revolves at a slightly different speed. Such centrifuges are capable of continuously receiving feed in the bowl and of separating the feed into layers of light and heavy phase materials (e.g. liquids and solids) which are discharged separately from the bowl. The screw conveyor structure, rotating at a differential speed with respect to the bowl, moves or xe2x80x9cscrollsxe2x80x9d an outer layer of heavy phase or solids slurry material to a discharge port or ports usually located in a tapered or conical end portion of the bowl. Centrifugal force tends to make the light phase material discharge through one or more ports usually located at an opposite end of the bowl. Typically the bowl is solid. Some bowls have Port(s) to reject the heavier solids phases.
Centrifugal separation results, preferably, in a discharge containing light phase material with little or no heavy phase material, and heavy phase material containing only a small amount of light phase material. When the light phase material is water and the heavy phase material contains soft solids, it is preferred that fairly dry solids and clean water be separately discharged.
Many different industries use decanter centrifuges in varied applications. They are used in the oil industry to process drilling mud to separate undesired drilling solids from the liquid mud. Some decanter centrifuges, because of their continuous operation, have the advantage of being less susceptible to plugging by solids. Also, they may be shut down for long or short periods of time and then restarted with minimum difficulty, unlike certain centrifuges which require cleaning to remove dried solids. Often the solids/liquid mixture is processed at extraordinarily high feed rates. To accommodate such feed rates, high torques are encountered, much energy is required to process the mixture, and the physical size of the centrifuge can become enormous.
As larger feed volumes are processed in a given centrifuge machine, the clarification capability of the centrifuge decreases due to decreased retention or residence time, partial-acceleration or nonacceleration (slippage) of the feed fluid (the solids/liquid mixture), radial deceleration of the fluid moving through the conveyor, and turbulence created by the movement and/or focusing of large volumes of fluid through ports that tend to transmit and/or focus a high volume flow in an area exterior to the conveyor that induces undesirable turbulence in that area and results in excess wear and abrasion to parts that are impacted by this flow. The turbulent fluid exiting from the ports impedes or prevents solids from flowing to solids exit ports and ports near the centrifuge""s drainage deck or xe2x80x9cbeachxe2x80x9d impedes solids flow up the beach.
FIG. 1 shows one typical prior art decanting centrifuge that removes free liquid from separated solids. A rotating bowl creates very high G-forces and forms a liquid pool inside the bowl. The free liquid and finer solids flow towards the larger end of the centrifuge and are removed through effluent overflow weirs. Larger solids settle against the bowl wall, forming a cake. These solids are pushed by a screw conveyor up out of the pool and across a drainage deck (conical section), or xe2x80x9cbeachxe2x80x9d. Dewatering or drying takes place during the process of the solids moving up the beach, with the deliquified solids discharged through a series of underflow solids ports. A gear box connects the conveyor to the bowl, causing the conveyor to rotate in the same direction as the bowl, but at a slightly different speed. This speed differential is required to convey and discharge solids.
The interior end of the feed tube is relatively close to a wall or member defining an end of an acceleration chamber, thus fluid exiting from the feed tube into the acceleration chamber has relatively little space in which to slow down. This relatively high speed fluid is, therefore, turbulent and can wear away parts of the acceleration chamber. Also exiting from the acceleration chamber via exit ports this turbulent-relatively-high-speed fluid can inhibit the desired flow of separated solids both in the bowl toward the solids exit ports and toward the beach area and can wear away parts of the conveyor and bowl adjacent the acceleration chamber exit ports. Rather than dispersing and slowing down the fluid exiting from the acceleration chamber, the exit ports focus and/or speed up the fluid flow.
The present invention, in certain aspects, discloses a new decanting centrifuge which has a rotatable bowl within which rotates a caged conveyor at a different speed than the speed of rotation of the bowl. In certain aspects a caged or skeleton conveyor according to the present invention includes a plurality of spaced-apart flights within which and to which are secured a plurality of spaced-apart support beams, rods, or members so that fluid can flow freely with reduced turbulence between the beams, rods or members, into and out from the interior of the conveyor. The flights form a screw portion of the conveyor for conveying solids separated from fluid to be treated by the centrifuge from one end of the bowl to the other (at which there are one or more solids outlets). In one aspect the flights are in the form of a helix.
The present invention, in certain aspects, provides a decanting centrifuge with a relatively short feed tube or inlet nozzle (providing a larger or longer area for reduction of fluid velocity, reduction of feed tube vibration, and turbulence reduction) and one or more impeller""s on the conveyor""s interior which are impacted by fluid entering the centrifuge through the feed tube or inlet nozzle. In certain aspects the impellers (and related parts such as a nose member, chamber, and base) are made of material from the group of steel, stainless steel, hardfaced or carbide covered metal, plastic, molded poly urethane, fiberglass, polytetrafluoroethylene, aluminum, aluminum alloy, zinc, or zinc alloy, stellite, nickel, chrome, boron and/or alloys of any of these. The impellers (and related parts) may be removable and/or replaceable. Any part of a conveyor or centrifuge disclosed herein, especially parts exposed to fluid flow, may be coated with a protective coating, hardfaced, and/or covered with tungsten carbide or similar material.
A xe2x80x9cvelocity decreasexe2x80x9d chamber or area, in certain embodiments, is, optionally, located past the nozzle (feed tube) (e.g. to the right of the interior end of the feed tube in FIGS 2A, 2B and 5Axe2x80x2, 5Bxe2x80x3. This unobstructed area may include space within a chamber.(e.g. within a solid-walled hollow member open at both ends) disposed between the feed tube exit and either conveyor fluid exit areas or, a radial acceleration apparatus within the conveyor. Fluid from the nozzle (e.g. two to two-and-one-half inches in internal diameter) moves through a chamber that disperses flowing fluid; provides a space to allow the fluid""s velocity to decrease (velocity in the general direction of the horizontal or longitudinal axis of the centrifuge); and directs fluid to impact the impellers. Different interchangeable nozzles may be used. The nozzle exit end may be non-centrally located within the conveyorxe2x80x94i.e. not on the conveyor""s longitudinal axis. A solid walled hollow member defining the chamber may be any suitable shapexe2x80x94e.g. but not limited to, conical, cylindrical, and/or triangular, square, rectangular, or polygonal in cross-section and any number of any known impellers, blades, or vanes may be used.
In certain embodiments fluid flows through the chamber and impacts a plurality of impellers that are connected to and rotate with the conveyor. The fluid impacts the impellers and is then moved radially outward by the blades toward the conveyor""s flights. The impellers are configured and positioned to radially accelerate the fluid so that as the fluid passes the impellers outer edges, the fluid""s speed (radial speed) is near or at the speed of a pool of material within the bowlxe2x80x94thus facilitating entry of this fluid into the pool or mass of fluid already in the bowl. By reducing or eliminating the speed differential between fluid flowing from the acceleration chamber and fluid already present in the bowl, turbulence is reduced, entry of solids of the entering fluid into the pool in bowl is facilitated, and more efficient solids separation results.
The present invention, in certain aspects, provides a centrifuge with a variable pneumatic backdrive or airbrake to control the differential speed of the conveyor. In one particular aspect a ROOTS XLP WHISPAIR blower available from the ROOTS DRESSER CO. is used to provide selectively variable braking for a gearbox pinion, thus varying the relative rotational speed of the conveyor in the bowl. In one aspect a typical known automatic boost system (e.g. to increase scroll-to-bowl speed or vice-versa) is used with the backdrive to inhibit or prevent plugging. Alternatively, for any embodiment herein the conveyor may be driven by a motor and a braking apparatus provided for the bowl to selectively adjust the conveyor/bowl rotative speed differential.
What follows are some of, but not all, the objects of this invention. In addition to the specific objects stated below for at least certain preferred embodiments of the invention, other objects and purposes will be readily apparent to one of skill in this art who has the benefit of this invention""s teachings and disclosures. It is, therefore, an object of at least certain preferred embodiments of the present invention to provide:
New, useful, unique, efficient, nonobvious devices and methods for decanting centrifuges;
Such centrifuges with dispersed and/or non-focused flow of fluid from an interior entry area, through a conveyor, into a bowl;
Such centrifuges with a caged or skeleton conveyor;
Such centrifuges with reduced fluid turbulence, particularly at points or areas at which fluid exits a conveyor to enter a bowl;
Such centrifuges with a relatively short feed tube and/or one or more impellers impacted by fluid entering the centrifuge through a feed tube and/or with a chamber for dispersing fluid flow and/or to reduce its longitudinal velocity for directing fluid flow to the impellers);
Such centrifuges with a pneumatic backdrive to adjust and control conveyor speed or bowl speed; and
Such centrifuges which effect increased settling and separation of solids.
Certain embodiments of this invention are not limited to any particular individual feature disclosed here, but include combinations of them distinguished from the prior art in their structures and functions. Features of the invention have been broadly described so that the detailed descriptions that follow may be better understood, and in order that the contributions of this invention to the arts may be better appreciated. There are, of course, additional aspects of the invention described below and which may be included in the subject matter of the claims to this invention. Those skilled in the art who have the benefit of this invention, its teachings, and suggestions will appreciate that the conceptions of this disclosure may be used as a creative basis for designing other structures, methods and systems for carrying out and practicing the present invention. The claims of this invention are to be read to include any legally equivalent devices or methods which do not depart from the spirit and scope of the present invention.
The present invention recognizes and addresses the previously mentioned problems and long-felt needs and provides a solution to those problems and a satisfactory meeting of those needs in its various possible embodiments and equivalents thereof. To one skilled in this art who has the benefits of this invention""s realizations, teachings, disclosures, and suggestions, other purposes and advantages will be appreciated from the following description of preferred embodiments, given for the purpose of disclosure, when taken in conjunction with the accompanying drawings. The detail in these descriptions is not intended to thwart this patent""s object to claim this invention no matter how others may later disguise it by variations in form or additions of further improvements.